The adult human has about 1014 microorganisms that normally reside in the digestive tract called the gut flora or intestinal bacteria. The bacteria that make up the normal flora of the digestive tract have a symbiotic or mutualistic relationship with their human hosts by performing a variety of beneficial functions such as breaking down and aiding in the absorption of otherwise undigestible food, stimulating cell growth, repressing the growth of harmful bacteria, training the immune system to respond to pathogens, and defending against some diseases.
As an important example, without the gut flora, humans would be unable to break down and use some of the carbohydrates they consume, because some species of intestinal bacteria have enzymes that human cells lack for breaking down polysaccharides. As another example, intestinal bacteria play an important role in promoting the early development and later functioning of the gut's mucosal immune system. These bacteria stimulate lymphoid tissue associated with the gut mucosa to produce antibodies to pathogens. During this process, the immune system is trained to recognize harmful bacteria, while leaving helpful species of bacteria unaffected, a tolerance developed in infancy.
The human infant is born with a digestive tract essentially free of bacteria. However, soon after birth, bacteria begin colonizing an infant's digestive tract. The first bacteria to be established in the gut affect the immune response making it more favorable to their own survival and less favorable to competing species; thus the first bacteria to colonize the gut are important in determining the person's lifelong gut flora makeup.
Among the bacterial genera found in the gut are Bacteroides, Clostridium, Fusobacterium, Eubacterium, Ruminococcus, Peptococcus, Peptostreptococcus, Bifidobacterium, Escherichia, and Lactobacillus, among others. Among these species, Bifidobacteria are noteworthy because it has recently been found that this genera of bacteria is responsible for digesting unique oligosaccharides found in human milk. Estimates indicate that about 40-97% of these oligosaccharides pass through the human infant digestive tract undigested. It has been suggested that these oligosaccharides found in human milk serve as a prebiotic: a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or the activity of one or a limited number of bacteria in the gastrointestinal tract. Thus, the one function of the oligosaccharides in human milk is to promote the colonization of the infant gut by beneficial bacteria such as Bifidobacteria. 
Cow's milk and commercially available formulas, however, are lacking the oligosaccharides found in human breast milk. Because of the widespread use of cow's milk and commercially available formulas in the feeding of new born infants, it would be beneficial to provide infants with a source of human milk oligosaccharides which are lacking in these forms of nourishment. Other classes of individuals such as the elderly or patients treated with certain antibiotics and others who have compromised gut flora would also benefit.